The present invention relates to the art of producing a purified grade of benzene.
Benzene is ordinarily purified by fractional distillation. Distillation, however, cannot remove certain unsaturated impurities such as diolefins which have a boiling point very close to that of benzene. Benzene which has only been fractionally distilled does not ordinarily have an acid-wash number of 2 or less.
Acid-wash color is a widely used measure of the level of unsaturated impurities in benzene. The test for acid-wash color is described in ASTM D-848-81. The test involves mixing sulfuric acid with benzene and shaking the mixture so that sulfuric acid will react with unsaturated impurities in the benzene. The discoloration caused by the product of sulfuric acid and the unsaturated impurities is the acid-wash color, which is measured and translated into a numerical assessment of discoloration. "Nitration Grade" benzene must have an acid-wash number of 2 or less.
Olefins have long been removed by passing the benzene through a clay treatment bed. The purification processes using clay treatment beds are described in several U.S. patents: Fukuda et al., Production of High Purity Benzene from Cracked Petroleum Residues, U.S. Pat. No. 3,400,168 (Sept. 3, 1968); Davis, Purification of Aromatic Hydrocarbons, U.S. Pat. No. 3,485,884 (Dec. 23, 1969); Ehrlich et al., Hydrodealkylation, U.S. Pat. No. 3,754,045 (Aug. 21, 1973); Vickers, Aromatics Extraction and Distillation Process, U.S. Pat. No. 4,070,408 (Jan. 24, 1978); and Takahashi et al., Process for Producing High Purity Benzene, U.S. Pat. No. 4,247,729 (Jan. 27, 1981). Clay beds are very inefficient from an industrial standpoint. They operate at high temperatures and high back pressures, are toxic hazards for persons exposed to the spent clay, and cannot be regenerated so that used clay must be landfilled in an environmentally sound manner.
Recent patents have also suggested that olefins can be removed by passing benzene over a hydrogenation catalyst with hydrogen prior to fractional distillation. In that manner, olefin impurities are hydrogenated to paraffins, which are easier to distill from the benzene. Patents which have discussed such a process include: Love et al., Production of Hydrocarbon Solvents of Low Acid-Wash Color, U.S. Pat. No. 2,731,506 (Jan. 17, 1956); Fukuda et al., Process for Producing High Purity Benzene, U.S. Pat. No. 3,310,592 (Apr. 19, 1965); Nelson et al., Method for Improving the Quality of Dealkylated Aromatic Compounds, U.S. Pat. No. 3,310,593 (Mar. 21, 1967); Peterson et al., Method for Improving the Quality of Aromatic Compounds, U.S. Pat. No. 3,310,594 (Mar. 21, 1967); Horne et al., Benzene from Pyrolysis Gasoline, U.S. Pat. No. 3,625,879 (Dec. 7, 1971); Eisenlohr et al., Production of Extra Pure Aromatics, U.S. Pat. No. 3,869,377 (Mar. 4, 1975); and Derrien et al., Process for Producing Benzene by Hydrodealkylation of a Hydrocarbon Fraction Comprising Alkyl-Aromatic Hydrocarbons, Olefinic Hydrocarbons and Sulfur Compounds, U.S. Pat. No. 4,463,206 (July 31, 1984). Foreign sources which describe catalytic hydrogenation to purify aromatic materials include: Essler, East German Patent No. 129,269 (published Jan. 11, 1978); Sumitomo Chemical, Japanese Patent No. 52/136,205 (published Nov. 14, 1977); and Mitsui Toatsu Chemical, Inc., Japanese Patent No. 60/185,733-A (published Sept. 21, 1985).
Those processes may effectively remove olefin impurities from benzene. However, other impurities found in the benzene can deactivate the catalyst, necessitating frequent replacement of the catalyst. Furthermore, such processes may be wasteful of energy. The benzene must ordinarily be heated up to a temperature appropriate for a gas-phase hydrogenation, then cooled back to a liquid phase to prepare for fractional distillation, and then heated and cooled again in the distillation process. What is needed is a process to purify benzene and drive its acid-wash color down to 2 or less without generating substantial amounts of environmentally hazardous by-products and without the need to repeatedly heat the benzene to a gas phase and then cool it to a liquid phase.